A variety of physical conditions involve two tissue surfaces that, for diagnosis or treatment of the condition, need to be separated or distracted from one another and then supported in a spaced-apart relationship. Such separation or distraction may be to gain exposure to selected tissue structures, to apply a therapeutic pressure to selected tissues, to return or reposition tissue structures to a more normal or original anatomic position and form, to deliver a drug or growth factor, to alter, influence or deter further growth of select tissues or to carry out other diagnostic or therapeutic procedures. Depending on the condition being treated, the tissue surfaces may be opposed or contiguous and may be bone, skin, soft tissue, or a combination thereof.
One such a condition that occurs in the orthopedic field is vertebral compression fractures. Vertebral compression fractures affect a significant part of the population, and add significant cost to the health care system. A vertebral compression fracture is a crushing or collapsing injury to one or more vertebrae. Vertebral fractures are generally but not exclusively associated with osteoporosis, metastasis, and/or trauma. Osteoporosis reduces bone density, thereby weakening bones and predisposing them to fracture. The osteoporosis-weakened vertebrae  can collapse during normal activity and are also more vulnerable to injury from shock or other forces acting on the spine. In severe cases of osteoporosis, actions as simple as bending forward can be enough to cause a vertebral compression fracture. Vertebral compression fractures are the most common type of osteoporotic fractures according to the National Institute of Health.
The mechanism of such vertebral fractures is typically one of flexion with axial compression where even minor events can cause damage to the weakened bone. While the fractures may heal without intervention, the crushed bone may fail to heal adequately. Moreover, if the bones are allowed to heal on their own, the spine may be deformed to the extent the vertebrae were compressed by the fracture. Spinal deformity may lead to breathing and gastrointestinal complications, and adverse loading of adjacent vertebrae.
Vertebral fractures happen most frequently at the thoracolumbar junction, with a relatively normal distribution of fractures around this point. Vertebral fractures can permanently alter the shape and strength of the spine. Commonly, they cause loss of height and a humped back. This disorder (called kyphosis or “dowager's hump”) is an exaggeration of the spinal curve that causes the shoulders to slump forward and the top of the back to look enlarged and humped. In severe cases, the body's center of mass is moved further away from the spine resulting in increased bending moment on the spine and increased loading of individual vertebrae.
Another contributing factor to vertebral fractures is metastatic disease. When cancer cells spread to the spine, the cancer may cause destruction of part of the vertebra, weakening and predisposing the bone to fracture.
Osteoporosis and metastatic disease are common root causes leading to vertebral fractures, but trauma to healthy vertebrae can also cause fractures ranging from minor to severe. Such trauma may result from a fall, a forceful jump, a car accident, or any event that compresses or otherwise stresses the spine past its breaking point. The resulting fractures typically are compression fractures or burst fractures.
Vertebral fractures can occur without pain. However, they often cause a severe “band-like” pain that radiates from the spine around both sides of the body. It is commonly believed that the source of acute pain in compression fractures is the result of instability at the fracture site, allowing motion that irritates nerves in and  around the vertebrae.
Until recently, treatment of vertebral compression fractures has consisted of conservative measures including rest, analgesics, dietary, and medical regimens to restore bone density or prevent further bone loss, avoidance of injury, and bracing. Unfortunately, the typical patient is an elderly person. As a class of patients, the elderly generally do not tolerate extended bed rest well. As a result, minimally invasive surgical methods for treating vertebral compression fractures have recently been introduced and are gaining popularity.
One technique used to treat vertebral compression fractures is injection of bone filler into the fractured vertebral body. This procedure is commonly referred to as percutaneous vertebroplasty. Vertebroplasty involves injecting bone filler (for example, bone cement, allograph material or autograph material) into the collapsed vertebra to stabilize and strengthen the crushed bone.
In vertebroplasty, physicians typically use one of two surgical approaches to access thoracic and lumbar vertebral bodies: transpedicular or extrapedicular. The transpedicular approach involves the placement of a needle or wire through the pedicle into the vertebral body, and the physician may choose to use either a unilateral access or bilateral transpedicular approach. The extrapedicular technique involves an entry point through the posterolateral corner of the vertebral body.
Regardless of the surgical approach, the physician generally places a small diameter guide wire or needle along the path intended for the bone filler delivery needle. The guide wire is advanced into the vertebral body under fluoroscopic guidance to the delivery point within the vertebra. The access channel into the vertebra may be enlarged to accommodate the delivery tube. In some cases, the delivery tube is placed directly into the vertebral body and forms its own opening. In other cases, an access cannula is placed over the guide wire and advanced into the vertebral body. After placement, the cannula is replaced with the delivery tube, which is passed over the guide wire or pin. In both cases, a hollow needle or similar tube is placed through the delivery tube into the vertebral body and used to deliver the bone filler into the vertebra.
In this procedure, the use of lower viscosity bone filler and higher injection pressures tend to disperse the bone filler throughout the vertebral body. However, such procedures dramatically increase the risk of bone filler extravasation  from the vertebral body. The transpedicular approach requires use of a relatively small needle (generally 11 gauge or smaller). In general, the small diameter needle required for a transpedicular approach necessitates injecting the bone filler in a more liquid (less viscous) state. Further, the pressure required to flow bone filler through a small gauge needle is relatively high. The difficulty of controlling or stopping bone filler flow into injury-sensitive areas increases as the required pressure increases. In contrast, the extrapedicular approach provides sufficient room to accommodate a larger needle (up to about 6 mm internal diameter in the lumbar region and lower thoracic regions). The larger needle used in the extrapedicular approach allows injection of bone filler in a thicker, more controllable viscous state. Therefore, many physicians now advocate the extrapedicular approach so that the bone filler may be delivered through a larger cannula under lower pressure. However, the transpedicular approach is still the preferred approach. Caution, however, must still be taken to prevent extravasation, with the greatest attention given to preventing posterior extravasation because it may cause spinal cord trauma. Physicians typically use repeated fluoroscopic imaging to monitor bone filler propagation and to avoid flow into areas of critical concern. If a foraminal leak results, the patient may require surgical decompression and/or suffer paralysis.
Another type of treatment for vertebral fractures is known as Kyphoplasty. Kyphoplasty is a modified vertebral fracture treatment that uses one or two balloons, similar to angioplasty balloons, to attempt to reduce the fracture and, perhaps, restore some vertebral height prior to injecting the bone filler. One or two balloons are typically introduced into the vertebra via bilateral transpedicular cannula. The balloons are inflated to reduce the fracture. After the balloon(s) are deflated and removed, leaving a relatively empty cavity, bone cement is injected into the vertebra. In theory, inflation of the balloons may restore some vertebral height. However, in practice it is difficult to consistently attain meaningful and predictable height restoration. The inconsistent results may be due, in part, to the manner in which the balloon expands in a compressible media, such as the cancellous tissue within the vertebrae, and the structural orientation of the trabecular bone within the vertebra, although there may be additional factors as well.
Thus there is a need for devices and methods to treat the above mentioned diseases, in particular compression vertebral fractures.
Another condition that can be treated by distraction or separation of  tissue layers is disruption or degeneration of an intervertebral disk. An intervertebral disk is made up of strong connective tissue which holds one vertebra to the next and acts as a cushion between vertebras. The disk is divided into two distinct regions: the nucleus pulposus and the annulus fibrosus. The nucleus lies at the center of the disk and is surrounded and contained by the annulus. The annulus contains collagen fibers that form concentric lamellae that surround the nucleus. The collagen fibers insert into the endplates of the adjacent vertebral bodies to form a reinforced structure. Cartilaginous endplates are located at the interface between the disk and the adjacent vertebral bodies.
Proper disk height is necessary to ensure proper functionality of the intervertebral disk and spinal column. The disk serves several functions, although its primary function is to facilitate mobility of the spine. In addition, the disk provides for load bearing, load transfer and shock absorption between vertebral levels. The weight of the person generates a compressive load on the disks, but this load is not uniform during typical bending movements. During forward flexion, the posterior annular fibers are stretched while the anterior fibers are compressed. In addition, a translocation of the nucleus occurs as the center of gravity of the nucleus shifts away from the center and towards the extended side.
Changes in disk height can have both local and broader effects. On the local (or cellular) level, decreased disk height results in increased pressure in the nucleus, which can lead to a decrease in cell matrix synthesis and an increase in cell necrosis and apoptosis. In addition, increases in intra-diskal pressure create an unfavorable environment for fluid transfer into the disk, which can cause a further decrease in disk height.
Decreased disk height may also result in significant changes in the overall mechanical stability of the spine. With decreasing height of the disk, the facet joints bear increasing loads and may undergo hypertrophy and degeneration, and may even act as a source of pain over time. Increased stiffness of the spinal column and decreased range of motion resulting from loss of disk height can lead to further instability of the spine, as well as back pain. Radicular pain may result from a decrease in foraminal volume caused by decreased disk height. Specifically, as disk height decreases, the volume of the foraminal canal, through which the spinal nerve roots pass, decreases. This decrease may lead to spinal nerve impingement, with associated radiating pain and dysfunction. 
Finally, adjacent segment loading increases as the disk height decreases at a given level. The disks that must bear additional loading are susceptible to accelerated degeneration and compromise, which may eventually propagate along the destabilized spinal column.
In spite of all of these detriments that accompany decreases in disk height, where the change in disk height is gradual many of the ill effects may be “tolerable” to the spine and may allow time for the spinal system to adapt to the gradual changes. However, a sudden decrease in disk volume caused by surgical removal of the disk or disk nucleus can heighten the local and global problems noted above.
The many causes of disruption or degeneration of the intervertebral disk can be generally categorized as mechanical, genetic and biochemical. Mechanical damage can include herniation in which a portion of the nucleus pulposus projects through a fissure or tear in the annulus fibrosus. Genetic and biochemical causes can result in changes in the extracellular matrix pattern of the disk and a decrease in biosynthesis of extracellular matrix components by the cells of the disk. Degeneration is a progressive process that usually begins with a decrease in the ability of the extracellular matrix in the central nucleus pulposus to bind water due to reduced proteoglycan content. With a loss of water content, the nucleus becomes desiccated resulting in a decrease in internal disk hydraulic pressure that ultimately results in a loss of disk height. This loss of disk height can cause non-tensile loading and buckling of the annulus. The loss of disk height also causes the annular lamellae to delaminate, resulting in annular fissures and rupture of the annulus. Herniation may then occur as rupture leads to protrusion of the nucleus.
Many disk defects are treated through a surgical procedure, such as a diskectomy in which the nucleus pulposus material is removed. During a total diskectomy, a substantial amount (and usually all) of the volume of the nucleus pulposus is removed and immediate loss of disk height and volume can result. Even with a partial diskectomy, loss of disk height can ensue.
Diskectomy alone is the most common spinal surgical treatment. The procedure is frequently used to treat radicular pain resulting from nerve impingement by a disk bulge or disk fragments contacting the spinal neural structures.
In another common spinal procedure, the diskectomy may be followed  by an implant procedure in which a prosthesis is introduced into the cavity left in the disk space after the nucleus material is removed. Thus far, the most prominent prosthesis is a mechanical device or a “cage” that is sized to restore the proper disk height and is configured for fixation between adjacent vertebrae. These mechanical solutions take on a variety of forms including solid kidney-shaped implants, hollow blocks filled with bone growth material, and threaded cylindrical cages.
A challenge of inserting a disk implant posteriorly is that a device large enough to contact the endplates and slightly expand the intervertebral space between the endplates must be inserted through a limited space. This challenge is often further heightened by the presence of posterior osteophytes, which may cause converging or “fish mouthing” of the posterior endplates that results in very limited access to the disk. A further challenge in degenerative disk spaces is the tendency of the disk space to assume a lenticular shape, which requires a relatively larger implant that often is not easily introduced without causing trauma to the nerve roots. The size of rigid devices that may safely be introduced into the disk space is thereby limited.
Cages of the prior art have been generally successful in promoting fusion and approximating proper disk height. Cages inserted from the posterior approach, however, are limited in size by the interval between the nerve roots. Some examples of prior art devices are shown in U.S. Pat. No. 5,015,247 to Michelson, which describes an artificial threaded spinal fusion implant; U.S. patent application Ser. No. 10/999,727 to Foley et al., which describes vertebral spacer devices for repairing damaged vertebral disks; U.S. Pat. No. 4,309,777 to Patil, which describes a motion preserving implant that has spiked outer surfaces to resist dislocation and contains a series of springs to urge the vertebrae away from each other; and finally, U.S. patent application Ser. No. 10/968,425 to Enayati, which describes an expandable intervertebral prosthesis. All the above patents and patent applications are hereby incorporated herein by reference.
Therefore, a need remains for a device that can be inserted into an intervertebral disk space in a minimally invasive procedure and is large enough to contact and separate adjacent vertebral endplates. There also remains a need for a device that reduces potential trauma to the nerve roots and still allows restoration of disk space height.
Another related area in which tissue distraction may be required is  spinal fusion. Fusion is a surgical technique in which one or more of the vertebrae of the spine are united together (“fused”) so that motion no longer occurs between them. In spinal fusion surgery, bone grafts are placed around the spine, and the body then heals the grafts over several months—similar to healing a fracture—and joins or “fuses” the vertebrae together.
There are many potential reasons for a surgeon to consider fusing vertebrae, such as treatment of fractured (broken) vertebra, correction of deformity (spinal curves or slippages), elimination of pain from painful motion, treatment of instability, and treatment of some cervical disk herniations.
One of the more common reasons to conduct spinal fusion is to treat a vertebral fracture. Although not all spinal fractures need surgery, some fractures, particularly those associated with spinal cord or nerve injury, generally require fusion as part of the surgical treatment. Certain types of spinal deformity, such as scoliosis, also are commonly treated with spinal fusion. Scoliosis is an “S” shaped curvature of the spine that sometimes occurs in children and adolescents. Fusion can be used as a form of treatment for very large curves or for progressively worsening smaller curves. Additionally, fusion can be used to treat spondylolisthesis, which is a condition that occurs when hairline fractures allow vertebrae to slip forward on top of each other.
Another condition that is treated by fusion surgery is actual or potential instability. Instability refers to abnormal or excessive motion between two or more vertebrae. It is commonly believed that instability can either be a source of back or neck pain or cause potential irritation or damage to adjacent nerves. Although there is some disagreement on the precise definition of instability, many surgeons agree that definite instability of one or more segments of the spine can be treated by fusion.
Cervical disk herniations that require surgery usually need removal of the herniated disk (diskectomy) and fusion. With this procedure, the disk is removed through an incision in the front of the neck (anteriorly) and a small piece of bone is inserted in place of the disk. Although disk removal is commonly combined with fusion in the neck, this is not generally true in the low back (lumbar spine).
Spinal fusion is also sometimes considered in the treatment of a painful spinal condition without clear instability. A major obstacle to the successful treatment of spine pain by fusion is the difficulty in accurately identifying the source  of a patient's pain. The theory is that pain can originate from painful spinal motion, and fusing the vertebrae together to eliminate the motion will eliminate the pain.
There are many surgical approaches and methods to fuse the spine, and they all involve placement of a bone graft between the vertebrae. The spine may be approached and the graft placed either from the back (posterior approach), from the front (anterior approach) or by a combination of both. In the neck, the anterior approach is more common and in the lumbar and thoracic regions a posterior approach is usually employed.
The ultimate goal of fusion is to obtain a solid union between two or more vertebrae. Fusion may or may not involve the use of supplemental hardware (instrumentation), such as plates, rods, screws and cages. Instrumentation can sometimes be used to correct a deformity, but it usually is just used as an internal splint to hold the vertebrae together while the bone grafts heal. Whether or not hardware is used, bone or bone substitutes are commonly used to get the vertebrae to fuse together. The bone may be taken either from another bone in the patient (autograft) or from a bone bank (allograft).
Yet another related area in which tissue distraction may be required is in the replacement of essentially an entire or a partially removed vertebra. Such removal is generally necessitated by extensive vertebral fractures, or tumors, and is not usually associated with the treatment of disk disease. Vertebral bodies may be compromised due to disease, defect, or injury. In certain cases, it becomes necessary to remove or replace one or more of the vertebral bodies or disks to alleviate pain or regain spinal functionality.
In the treatment of a removed vertebra, a device is used to form a temporary structural mechanical support that aids in replacing the removed vertebra with bone filler, such as calcium phosphate which promotes healing. A number of methods and devices have been disclosed in the prior art for replacing a diseased or damaged vertebral body. These prior art devices and the procedures associated therewith have difficulty in maintaining the proper structural scaffolding while a castable material, such as bone cement, is hardened in the cavity left by the removed vertebral body. The maintaining of proper structural scaffolding has been especially difficult in a minimally invasive posterior surgical approaches.
Spinal fusion or lumbar spinal fusion is one way to treat a compromised vertebral body due to unstable burst fractures, severe compression fractures, and  tumor decompression. In a spinal fusion procedure, the disks above and below the compromised vertebral body are removed and a strut graft and plate are then used to make the vertebrae above and below the replaced vertebral body grow together and become one bone.
Some of the prior art vertebral body replacement systems include U.S. Pat. No. 6,086,613 to Camino et al., which describes an interbody fusion system made of a titanium mesh and endplates; U.S. Pat. No. 5,192,327 to Brantigan, which describes the use of singular or stackable modular implants; U.S. Pat. No. 6,585,770 to White et al., which describes a hollow body with an opening to receive bone growth inducing material; and U.S. Pat. No. 6,758,862 to Berry, which describes a vertebral replacement body device. All of the aforementioned references are hereby incorporated herein by reference.
Thus, there remains a need for improved devices for replacing one or more removed or partially removed vertebral bodies especially from a posterior approach and in a minimally invasive surgical intervention.